Models for the ultimate development of effective vaccines and immunotherapies that would limit HIV replication can be drawn from naturally occurring examples of immune system-mediated control. Identifying the components, targets, and magnitude of an effective immune response to HIV are important steps toward developing effective vaccines and immunotherapies. Although patients with normal CD4+ T cell counts and low levels of plasma virus are a heterogeneous group, a small subgroup of patients with truly non-progressive HIV infection and restriction of virus replication likely holds important clues to the basis of an effective cellular immune response to HIV. A small subpopulation of HIV-infected individuals (fewer than 0.8%) shows no signs of disease progression over a 10-year period. We have assembled a stringently defined cohort of such patients, termed long-term nonprogressors (LTNPs), or elite controllers. Many of these patients have been infected for 20 years, yet even without receiving antiretroviral therapy have experienced no CD4+ T-cell decline and have maintained plasma viral RNA levels below 50 copies per milliliter. We are using cells from these patients to systematically dissect the mechanisms of immune-mediated restriction of virus replication. The HIV-specific T-cell responses of these patients have been studied in extreme detail. Through this project, considerable progress has been made in understanding how the immune system controls HIV. Our prior work indicated that there is a dramatic association between immunologic control and the HLA B*5701 allele, and that the immune response is highly focused on peptides restricted by this allele. This result established both host genetic and functional links between immunologic control and the CD8+ T-cell responses of these patients. The finding of high frequencies of CD8+ T cells specific for the patients virus in both LTNPs and progressors strongly suggested that differences between responses of these patient groups were qualitative rather than quantitative in nature. One important qualitative difference in the HIV-specific immune response that distinguishes LTNPs from progressors is the maintenance of HIV-specific CD8+ T cells with a high proliferative capacity. This proliferation parallels perforin expression required for effective killing of HIV-infected CD4+ T cells. To better understand the mechanisms of immunologic control, we devised a method to measure HIV-infected cell killing on a per-cell basis. Measured on a per-cell basis, HIV-specific CD8+ T cells of LTNPs efficiently eliminated primary autologous HIV-infected CD4+ T cells. This effective killing was clearly distinguishable from the responses of progressors over a very broad range of effectors to HIV-infected targets. Progressor cells did not mediate effective killing even at high effector-to-target ratios. Defective cytotoxicity of progressor effectors could be restored in vitro. These results establish an effector function and a mechanism that clearly segregate with immunologic control of HIV. One widely held hypothesis for the impact of MHC in the control of lentiviral infection, is that these alleles mediate their protective effect by presenting peptides whose sequences are conserved due to structural or functional constraints on the virus. A better understanding of the contributions of epitope targeting and conservation could potentially be obtained by investigating features of the response shared between LTNP/EC with and without protective alleles. Depending upon the case definition used, 59-79% of LTNP/EC bear HLA B*27 or B*57. The CD8+ T-cell response of LTNP/EC without protective alleles had been anecdotally reported and not well characterized. We therefore analyzed the epitope specificities targeted in a cohort of B*27/57- LTNP/EC to provide greater insight into the mechanisms of control over HIV replication. Of immunodominant HIV-specific CD8+ T-cell responses of the B*27/57- patients in this study, more than 2/3 were not directed against the same or overlapping sequences as those restricted by B27 or B57. Epitopes targeted by HLA B*27/57- LTNP/EC are not differentiated by conservation. These data suggest that targeting areas containing B27- or B57-restricted peptides is not a requirement for immunologic control of HIV-1. Qualitative features of virus-specific CD8+ T-cells, such as proliferative and cytotoxic capacities, are by far the most robust parameters differentiating the responses of B*27/57+ LTNP/EC from those of progressors. We recently examined the cytotoxic capacity of HIV-specific CD8+ T cells from B*27/57- LTNP/EC against primary autologous HIV-infected targets. The HIV-specific CD8+ T-cell cytotoxic responses of B*27/57- LTNP/EC were high overall and comparable to those of B*27/57+ LTNP/EC. These cytotoxic responses were significantly higher than those of progressors, including some who were B*27/57+. These findings support that the feature of the HIV-specific CD8+ T cell response shared between B*27/57- and B*27/57+ LTNP/EC is both increased bulk and per-cell cytotoxicity compared to progressors. In summary, it is the HIV-specific cytotoxic function, not specificity, that is the feature shared among LTNP/EC across HLA types, and that differentiates them from progressors. In addition, cells from patients in this cohort are providing important clues regarding the basis of immunologic control or immunopathogenesis of HIV-1 in progressors. For example, recent work with collaborators has shown that the protective effect of HLA B*57 allele in the control of HIV-1 replication can be modified by the Killer cell immunoglobulin-like receptor 3DL1. In addition, NIAID collaborators have used cells from patients from this cohort to show an important role for MadCam costimulation in driving HIV replication in CD4+ T cells. Taken together, our work suggests that cytotoxic capacity is a robust correlate of immunologic control of HIV and SIV in chronic infection and may also be an important immune correlate in vaccinees. Over the coming years we are optimistic that this work will provide a long-sought after T cell correlate of immunity that will predict immunologic control of lentiviruses and, in the process, accelerate HIV vaccine development.